Tl;dr: The big question is, why are big animals dumber than humans when they can dedicate a very small portion of calories (as low as .01%) to achieve a great deal of intelligence greater than humans even?

Disclaimer: This is another post of mine speculating on the rare intelligence hypothesis of the fermi paradox. Most of the stuff in this post is just speculation and a lot of the numbers are pretty rough (maybe a sperm whale will eat 40 million calories or 4 million calories) but try to look at the broader picture here. I recommend reading the whole post before you post a rebuttal, especially the section on my "probably bad theories"

If you think about it pretty simply, the human brain consumes a certain amount of calories (I've heard roughly 400-500 calories). A portion of that goes to automatic functions like breathing or heart beating or hormone regulation or pain reception or smelling and so on. Another portion of that (let's say 50% so we can have a nice even 250 calories to higher brain functions) goes to higher functions like talking, strategic planning, etc. That means that human brain functions only require 250 calories to function, or I guess theoretically 500 calories as an absolute maximum. That's maybe 10% of their calories going to higher functions and that seperates us from Chimps who may use only 8% of their resting energy and consume less food too because they are smaller.

In contrast to this, a large whale like the mighty sperm whale consumes around 20 million calories a day to support itself. Even dedicating an extra measly .01% of your daily calories (for a human about the amount of calories in a bread crumb) to brain power would produce a brain that uses 100x as much energy as a normal human brain. Having a super mind that allows for strategic planning, superior social skills, etc seems like a huge advantage when it uses a very very tiny fraction of your daily energy intake. So that begs the question? Why don't sperm whales take advantage of this extreme return on investment? I have 3 main hypothesises:

1. Changing the Brain Structure Isn't a Great Idea/if it ain't Broke don't Fix it: It may be that intelligence is a difficult evolution path and requires extremely gradual and subtle changes to work unless there is a very very large selection bias for brain power. There are two main reasons for this: the first reason is that brains are pretty complex and screwing around with that structure via mutations has a very high chance of messing up the whole thing and resulting in miscarriage or death or intellectual disability for these whales. Secondly, increasing the number of brains cells could cause brain cancer more frequently. Having more brain cells in general just increases the odds of a brain cancer. So, if a sperm whale is doing fine in it's ecological niche there might not be a large selection pressure for large brains. Additionally, sperm whales don't really congregate in packs meaning there isn't much of a social incentive either. However, this is not the case in other large predatory cetaceans like orca whales so it clearly isn't a full explanation to say that "whales are solitary and there is no social selection pressure". It requires an explanation of why social pressures were more intense in humans than in orca whales.
2. Intelligence is an Evolutionary Dead End: This one has been discussed by Isaac Arthur and others a lot so I won't go as deep into it but it's possible that intelligence is a dead end. It's possible that mental illnesses that harm survival increase as intelligence increases due to a more complex brain structure meaning that more things can go wrong. For example, when in captivity, wild parrots pluck and exhibit traits like mental illness when neglected and sometimes when not neglected. This behavior is not observed in snakes or reptiles which are less intelligent animals. It could be that larger brains require greater socialization in order to not have the animal go crazy. Other things like maybe if a sperm whale gets too smart it gets too good at hunting its specialized cephalopod prey and the whales are probably the ones that go extinct due to their lower reproductive rate. Things like that might be difficult to differentiate in the fossil record. Maybe intelligence just isn't that useful either. We obviously value our intelligence but whales have a lot of blubber so they don't need to plan things in advanced and maybe their natural instincts aren't enhanced very much sapient intelligence or even just greater intelligence. You can propose any other ideas you might have about this in the comments.
3. They've Reached a Strange Evolutionary Trap: It could be that cetacean brains are configured in such a way that there is some barrier to intelligence that has developed because the brain can't be arranged in a certain way or can only use so much energy for whatever reason and requires drastic anatomical changes to support a different neural configuration or they need to evolve more efficient neurons because they have super inefficient ones. Imagine you're in a maze and take a path that says "exit" however the path is much longer than it should be and then eventually you hit a dead end. In order to get to the exit, you need to take a longer path than you originally would have needed, I don't think we know enough about brains to say much about this theory but the main reason I don't like it is because there are tons of other megafauna in nature that are both stupid and smart. They don't dedicate small amounts of calories to sapient intelligence. An elephant would need to dedicate 0.35% of it's calories but it just simply hasn't evolved sapience.
4. (Theory I consider bad #1) Whales are Already Intelligent: I consider this theory to be bad because it seems that this paradox applies to a ton of animals and not just whales. This paradox is almost equally as prescient with an elephant or rhino. Both are less intelligent than humans and with elephants especially they have a good degree of dexterity and are simply not great at tool use. Additionally, they can't do math or understand complex symbols or talk. I just haven't seen anything that demonstrates elephant intelligent. The same thing goes for cetaceans. This one is technically possible but I just don't see it.
5. (Theory I consider bad #2) Humans Were "Chosen" by Aliens: This theory states that aliens interbred with us or did something or other that made us more intelligent than the animals around us. I don't buy this theory mainly because of the continuity of fossil and genetic evidence present. We don't have 10% "alien" or "injected" DNA in our genomes that would indicate anything like this and the fossil record clearly demonstrates a gradual transition from less intelligent to more intelligent forms.

So with this said, what do you guys think?